[RFC PATCH 1/3] riscv: Remove 2MB offset in the mm layout

[Anup Patel]

+Alex

On Tue, Nov 23, 2021 at 7:27 AM <guoren at kernel.org> wrote:
>
> From: Guo Ren <guoren at linux.alibaba.com>
>
> The current RISC-V's mm layout is based on a 2MB offset and wasting
> memory. Remove 2MB offset and map PAGE_OFFSET at start_of_DRAM.
> Then we could reduce the memory reserved for opensbi in the next
> patch.

The real problem is that the generic kernel marks memory before
__pa(PAGE_OFFSET) as reserved which is evident from the boot
print "OF: fdt: Ignoring memory range 0x80000000 - 0x80200000".

One simple way to re-claim the first 2MB of memory is by:
1) Not placing OpenSBI firmware at start of RAM and rather
place it towards end/middle or RAM away from kernel and initrd
2) Load kernel at start of the RAM

The point#1 is already supported by OpenSBI firmwares using
position independent compilation. In fact, U-Boot SPL does
not load OpenSBI firmware at the start of RAM.

I would suggest Allwinner D1 to follow U-Boot SPL and have
the booting stage before OpenSBI to load OpenSBI firmware
somewhere else.

Regards,
Anup

>
> Signed-off-by: Guo Ren <guoren at linux.alibaba.com>
> Cc: Palmer Dabbelt <palmer at dabbelt.com>
> Cc: Anup Patel <anup.patel at wdc.com>
> Cc: Atish Patra <atishp at rivosinc.com>
> ---
>  arch/riscv/include/asm/page.h   |  8 ++++++++
>  arch/riscv/kernel/head.S        | 10 +++-------
>  arch/riscv/kernel/vmlinux.lds.S |  5 ++---
>  arch/riscv/mm/init.c            | 11 ++++++++---
>  4 files changed, 21 insertions(+), 13 deletions(-)
>
> diff --git a/arch/riscv/include/asm/page.h b/arch/riscv/include/asm/page.h
> index b3e5ff0125fe..299147c78b4a 100644
> --- a/arch/riscv/include/asm/page.h
> +++ b/arch/riscv/include/asm/page.h
> @@ -16,6 +16,14 @@
>  #define PAGE_SIZE      (_AC(1, UL) << PAGE_SHIFT)
>  #define PAGE_MASK      (~(PAGE_SIZE - 1))
>
> +#if __riscv_xlen == 64
> +/* Image load offset(2MB) from start of RAM */
> +#define LOAD_OFFSET    0x200000
> +#else
> +/* Image load offset(4MB) from start of RAM */
> +#define LOAD_OFFSET    0x400000
> +#endif
> +
>  #ifdef CONFIG_64BIT
>  #define HUGE_MAX_HSTATE                2
>  #else
> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> index f52f01ecbeea..a6ac892d2ccf 100644
> --- a/arch/riscv/kernel/head.S
> +++ b/arch/riscv/kernel/head.S
> @@ -61,13 +61,7 @@ ENTRY(_start)
>         /* Image load offset (0MB) from start of RAM for M-mode */
>         .dword 0
>  #else
> -#if __riscv_xlen == 64
> -       /* Image load offset(2MB) from start of RAM */
> -       .dword 0x200000
> -#else
> -       /* Image load offset(4MB) from start of RAM */
> -       .dword 0x400000
> -#endif
> +       .dword LOAD_OFFSET
>  #endif
>         /* Effective size of kernel image */
>         .dword _end - _start
> @@ -94,6 +88,8 @@ relocate:
>         la a1, kernel_map
>         XIP_FIXUP_OFFSET a1
>         REG_L a1, KERNEL_MAP_VIRT_ADDR(a1)
> +       li a2, LOAD_OFFSET
> +       add a1, a1, a2
>         la a2, _start
>         sub a1, a1, a2
>         add ra, ra, a1
> diff --git a/arch/riscv/kernel/vmlinux.lds.S b/arch/riscv/kernel/vmlinux.lds.S
> index 5104f3a871e3..75b7c72cd4bd 100644
> --- a/arch/riscv/kernel/vmlinux.lds.S
> +++ b/arch/riscv/kernel/vmlinux.lds.S
> @@ -11,10 +11,9 @@
>  #else
>
>  #include <asm/pgtable.h>
> -#define LOAD_OFFSET KERNEL_LINK_ADDR
>
> -#include <asm/vmlinux.lds.h>
>  #include <asm/page.h>
> +#include <asm/vmlinux.lds.h>
>  #include <asm/cache.h>
>  #include <asm/thread_info.h>
>  #include <asm/set_memory.h>
> @@ -32,7 +31,7 @@ PECOFF_FILE_ALIGNMENT = 0x200;
>  SECTIONS
>  {
>         /* Beginning of code and text segment */
> -       . = LOAD_OFFSET;
> +       . = LOAD_OFFSET + KERNEL_LINK_ADDR;
>         _start = .;
>         HEAD_TEXT_SECTION
>         . = ALIGN(PAGE_SIZE);
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 24b2b8044602..920e78f8c3e4 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -221,6 +221,11 @@ static void __init setup_bootmem(void)
>         if (!IS_ENABLED(CONFIG_BUILTIN_DTB))
>                 memblock_reserve(dtb_early_pa, fdt_totalsize(dtb_early_va));
>
> +       /*
> +        * Reserve OpenSBI region and depends on PMP to deny accesses.
> +        */
> +       memblock_reserve(__pa(PAGE_OFFSET), LOAD_OFFSET);
> +
>         early_init_fdt_scan_reserved_mem();
>         dma_contiguous_reserve(dma32_phys_limit);
>         if (IS_ENABLED(CONFIG_64BIT))
> @@ -604,7 +609,7 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>
>         kernel_map.va_kernel_xip_pa_offset = kernel_map.virt_addr - kernel_map.xiprom;
>  #else
> -       kernel_map.phys_addr = (uintptr_t)(&_start);
> +       kernel_map.phys_addr = (uintptr_t)(&_start) - LOAD_OFFSET;
>         kernel_map.size = (uintptr_t)(&_end) - kernel_map.phys_addr;
>  #endif
>         kernel_map.va_pa_offset = PAGE_OFFSET - kernel_map.phys_addr;
> @@ -645,8 +650,8 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>         create_pmd_mapping(trampoline_pmd, kernel_map.virt_addr,
>                            kernel_map.xiprom, PMD_SIZE, PAGE_KERNEL_EXEC);
>  #else
> -       create_pmd_mapping(trampoline_pmd, kernel_map.virt_addr,
> -                          kernel_map.phys_addr, PMD_SIZE, PAGE_KERNEL_EXEC);
> +       create_pmd_mapping(trampoline_pmd, kernel_map.virt_addr + LOAD_OFFSET,
> +                          kernel_map.phys_addr + LOAD_OFFSET, PMD_SIZE, PAGE_KERNEL_EXEC);
>  #endif
>  #else
>         /* Setup trampoline PGD */
> --
> 2.25.1
>